Heat issues are most likely curbing your units output.  When in a residential setting (open windows as cooling method) these units tend to stay around 1.2-1.3TH, dropping occasionally to 400GH/s to let internal temperature drop.  One of our client's units has the same problem as yours though, 1 core dead, we recently have just RMA'd the unit and should be getting a new one this week.

In our temperature controlled hosting facility running at 74 degrees F, with 30% humidity our Terraminer IV's are steady at 1.75TH/s.